A supramolecular peptide nanofiber templated Pd nanocatalyst for efficient Suzuki coupling reactions under aqueous conditions

Article abstract of DOI:10.1039/c2cc36228g

A bioinspired peptide amphiphile nanofiber template for formation of one-dimensional Pd nanostructures is demonstrated. The Pd and peptide nanocatalyst system enabled efficient catalytic activity in Suzuki coupling reactions in water at room temperature.

Full text of DOI:10.1039/c2cc36228g

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A supramolecular peptide nanofiber templated Pd nanocatalyst for
efficient Suzuki coupling reactions under aqueous conditionsw
Mohammad Aref Khalily,z Oya Ustahuseyin,z Ruslan Garifullin, Rukan Genc and
Mustafa O. Guler*
Received 27th August 2012, Accepted 4th October 2012
DOI: 10.1039/c2cc36228g
A bioinspired peptide amphiphile nanofiber template for formation
of one-dimensional Pd nanostructures is demonstrated. The Pd and
peptide nanocatalyst system enabled efficient catalytic activity in
Suzuki coupling reactions in water at room temperature. The
nanocatalyst system can be easily separated and reused in successive
reactions without significant loss in activity and structural integrity.
are of great interest because of their versatile chemical and
physical properties.13 A few studies previously reported
heterogeneous palladium catalysts, which were synthesized
by a nanoscale and environmentally friendly template for C–C
coupling reactions.14 Self-assembled peptide amphiphile (PA)
nanofibers are promising candidates as a template and support
owing to their tailorable surface properties. By employing
metal-binding amino acids in PA structure (e.g. lysine, glutamic
acid and DOPA), peptide nanofibers can specifically bind metal
ions for functional material applications.15,16 These bioinspired
peptide nanostructures can be further exploited for seeding and
nucleation of metal ions for producing nanoscale inorganic
nanostructures.15
Metal nanostructures are attractive catalysts for a wide variety of
organic reactions due to increased accessibility to surface atoms and
lower coordination numbers compared to bulk equivalents, which
result in enhanced catalytic activity.1 Palladium nanoparticles have
been used as catalysts for C–C coupling such as Suzuki and Heck
reactions as a suspension or adsorbed onto a support i.e. carbon
structures,2 dendrimers,3 polymers,4 metal–organic frameworks5
and mesoporous silica.6 In most cases, solid support materials
provide easy separation and enable reuse of the catalyst.
Catalytic activity of palladium nanoparticles is further enhanced
by employing nanoscale template directed synthesis providing
high surface area nanoparticles.7,8 However, not only the catalyst
performance is important, but also environmental concerns
should be taken into account during development of novel
catalysts.9 The research for environmentally friendly chemical
synthesis led to a search for catalytic reactions in cheap, readily
available, non-toxic, non-flammable and environmentally friendly
solvents such as water at room temperature.10 Suzuki–Miyaura
reactions in water with improved yield and a simple reaction
protocol still remain a major challenge. In general, various
additives such as phosphine ligands and quaternary ammonium
salts such as tetrabutylammonium bromide (TBAB) or cetyl-
trimethylammonium bromide (CTAB) are required for effective
reaction progress in aqueous media.11,12 These additives lower
the atom economy of the reaction and produce extra waste
products.
Herein, we report bioinspired supramolecular peptide nano-
fiber templated Pd0 hybrid nanocatalyst (Pd@Peptide) for mild
and efficient Suzuki–Miyaura coupling reactions in aqueous
media at room temperature without additives. The efficiency of
the Pd@Peptide nanocatalyst system was demonstrated by
successful application in a variety of reagents. In this study,
we designed and synthesized a de novo peptide amphiphile
molecule (Lauryl-VVAGHH-Am (Fig. S1, ESIw)) that can
coordinate to PdII ions through lone pair electrons in imidazole
moieties of histidine residues in the peptide (Scheme 1).17
Biological template materials (e.g. peptides, proteins, viruses,
and bacteria) in the synthesis of catalytic metal nanostructures
Institute of Materials Science and Nanotechnology, National
Nanotechnology Research Centre (UNAM), Bilkent University,
Ankara, Turkey 06800. E-mail: moguler@unam.bilkent.edu.tr;
Fax: +90 312 266 4365; Tel: +90 312 290 3552
w Electronic supplementary information (ESI) available: Experimental
details for synthesis and characterization of the peptide amphiphile
molecule and the Pd–peptide system. See DOI: 10.1039/c2cc36228g
z These authors contributed equally.
Scheme 1 (a) Self-assembly of peptide amphiphile molecules gener-
ates nanofibers with 10 nm diameter. Seeding and reduction of PdII
ions on the surface of peptide nanofibers form hybrid peptide and Pd0
nanostructures (Pd@Peptide). (b) Pd@Peptide nanostructures can be
used as catalyst in C–C coupling reactions.
c
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nanoparticles, because as grain size decreases, its XRD pattern
broadens (Fig. S9, ESIw). Suzuki coupling reactions were per-
formed as a model reaction to determine catalytic activity of the
Pd nanostructures on the peptide nanofibers. Water was used for
coupling reactions as an attractive green and cheap solvent.19 We
optimized Pd@Peptide loading as 1.5 mol% (Table S4, ESIw)
and reaction conditions for iodobenzene (Table S1, entry 1,
ESIw) in water at room temperature. The reaction was completed
in less than 4 h with 99% biphenyl conversion (Table S1, entry 1,
ESIw). Mixing ethanol, which is another environmentally friendly
solvent, expedited the reaction rate four folds (Table S1, entry 2,
ESIw). The rate enhancement is potentially due to improved
solubility of the starting materials in the water–ethanol mixture.20
Various aryl iodides were employed to investigate the diversity of
aryl iodides tolerated in Suzuki–Miyaura coupling reactions with
aryl boronic acids (Table S1, ESIw).
The Pd@Peptide nanocatalyst revealed excellent catalytic
activity towards diverse substrates under the optimized reac-
tion conditions (Table 1, entry 1). When activated aryl iodide
(1-fluoro-4-iodobenzene) was used instead of iodobenzene, the
biaryl product (Table 1, entry 2) was obtained in only 1 h in
99% yield. It should also be noted that the corresponding
biaryl product was obtained in only 2 h in quantitative yield
when deactivated 1-iodo-4-methoxybenzene (Table 1, entry 3)
was used. The Pd@Peptide nanocatalyst can activate 4-iodoanisol
efficiently. Replacing phenyl boronic acid with 40-methoxy-
biphenyl-4-ylboronic acid demonstrated interesting results.
40-methoxybiphenyl-4-ylboronic acid reacted with iodobenzene,
1-fluoro-4-iodobenzene (activated), and 1-iodo-4-methoxy-
benzene (deactivated) and yielded the desired products (Table 1,
entries 5, 6, and 7) in high yields in only 1 h. The electron-
withdrawing and electron-donating groups in aryl iodides did
not have considerable effect on the reaction rates. We tested
several conditions for aryl bromides as summarized in Table
S1 (ESIw). It was previously reported that activation of the
Br–C bond is more challenging than the I–C bond.21 Only a
trace amount of the biphenyl product was observed for
bromobenzene in water at room temperature (Table 1, entry 3)
and at 80 1C (Table 1, entry 4). The nature of the base is reported
to be crucial in different Suzuki coupling reactions.22 Therefore,
the base was changed from K3PO4 to K2CO3. However, it
did not improve the reactions for bromobenzene derivatives
(Table S1, entries 5 and 6, ESIw).
Fig. 1 (a) TEM image of the peptide nanofibers, (b) SEM image of
the peptide nanofiber network, (c) and (d) TEM images of Pd
nanoparticles on the peptide nanofibers.
At pH > 6, the PA molecules can self-assemble into
nanofibers with a diameter of ca.10 nm (Fig. 1a) directed by
b-sheet structures (Fig. S3, ESIw). A three-dimensional network of
the PA nanofibers forms a self-supporting hydrogel at concen-
tration of 1 wt% (Fig. 1b and Fig. S4–S6, ESIw). In this work, we
exploited peptide nanofibers as a nanoscale template for formation
of Pd0 nanoparticles. Following
a multi-step reduction
methodology, closely-packed one-dimensional palladium nano-
particles were grown on peptide nanofibers. PdII ions accumulated
on the peptide nanofibers due to their affinity to imidazole
residues. After peptide nanofiber formation at pH 7, palladium
solution was added to the peptide nanofiber hydrogel and the
mixture was left at room temperature overnight to enable
interaction of the ions with the imidazole moiety of histidine
residues in the peptide. Later, a reducing agent (L-(+)-ascorbic
acid) was added to the mixture. After the first reduction, the
PdII ion amount was increased for second and third reduction
cycles by increasing the Pd/peptide molar ratio. Nanoparticle
formation on the peptide template was inadequate in the first
reduction cycle (Fig. S7, ESIw). After the third cycle, coating of
peptide nanofibers with closely packed palladium nanoparticles
was clearly observed (Fig. 1c and d). To remove peptide
molecules without Pd, the sample was filtered through a
cellulose membrane with 0.2 mm cut off. Filtration was found
to be the most effective and easiest way of eliminating the excess
peptide without disturbing the integrity of the catalyst assembly.
To the best of our knowledge, this type of highly ordered Pd
nanostructures was obtained for the first time by use of a peptide
nanofiber template methodology.
Table 1 Suzuki–Miyaura coupling of aryl iodides with Pd@Peptide
nanocatalysta
Entry
R1
R2
Time (h)
Conversionb (%)
1
2
3
4
5
6
H
F
OMe
H
F
OMe
H
H
H
OMe
OMe
OMe
4
1
2
1
1
1
99
99
99
99
85
99
Metal nanoparticle loading capacity of the peptide nanofiber
templates was assessed by thermogravimetric analysis (TGA).
The Pd@Peptide sample was dried at 100 1C in an oven, and
heated to 550 1C. The inorganic content was found to be 42.5%
(Fig. S8, ESIw). The crystalline structure of the Pd@Peptide
sample was analyzed by X-ray diffractometry (XRD). The
dominant surface is {111}. Pd {111} is the lowest energy facet
and the most stable facet of Pd.18 The presence of a broad Pd
{111} pattern expresses low periodic and atomic order of Pd
a
Reaction conditions: aryl iodide (0.5 mmol), arylboronic acid
(0.75 mmol), Pd@Peptide (1.5 mol% with respect to aryl iodide
b
concentration), K3PO4 (2.0 equiv.), solvent (4 mL). The reaction
yield was determined by GC-MS (Fig. S21–S30, ESI).
c
Chem. Commun.
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Table 2 Suzuki–Miyaura coupling of aryl bromides with Pd@Peptide
nanocatalysta
ethanol–water mixture at 80 1C). The catalyst was reused in
consequent reactions and showed efficient catalytic activity
even after fifth use (95% product conversion, Table S3, ESIw).
No considerable change was observed in the structural integrity
of the nanocatalyst when the Pd@Peptide nanocatalyst was
treated under the same reaction conditions (Fig. S10, ESIw).
Developing efficient and green catalysts is important for new
technologies to eliminate waste, to avoid using hazardous
solvents and reagents, and to possess high recyclability. Here,
we demonstrated a bioinspired peptide amphiphile nanofiber
template for formation of one-dimensional Pd nanostructures.
The Pd@Peptide nanocatalyst system provided high catalytic
activity in Suzuki coupling reactions under environmentally
friendly conditions. Moreover, the nanocatalyst can be easily
isolated and reused at least 5 times in consecutive reactions
without significant loss in activity and structural integrity.
We believe that this novel approach can find applications in
many industrially important catalytic processes under environ-
mentally friendly conditions.
Entry
R1
R2
Time (h)
Conversionc (%)
1
H
H
H
H
H
OMe
4
24
12
24
4
99
99
99
99
83
2
NO2
NO2
OMe
H
3b
4
5
a
Reaction conditions: aryl bromide (0.5 mmol), arylboronic acid
(0.75 mmol), Pd@Peptide (1.5 mol% with respect to aryl bromide
concentration), NaOH (2.0 equiv.), water (5 mL) at room temperature.
b
c
10% ethanol was used. The reaction yield was determined by
GC-MS (Fig. S43–S50, ESI).
Inspired from the four fold rate enhancement in entry 2
(Table S1, ESIw), we mixed ethanol as a green, organic and
water miscible solvent with water. When ethanol was mixed
with water in a 1 : 1 ratio, some rate enhancement was
observed at room temperature (Table S1, entry 8, ESIw).
Increasing temperature to 80 1C yielded the desired product
in almost quantitative yield in less than 4 h. High activation of
the Br–C bond and low solubility of bromobenzene in water
caused low catalytic activity due to limited interaction between
the reactants. The Pd@Peptide nanocatalyst revealed high
catalytic activity in the water–ethanol mixture for Br–C bond
activation. Interestingly, most of the Suzuki coupling reactions
were completed in almost 2 h with very high conversions
regardless of the presence of electron donating and withdrawing
groups in the reactant molecules (Table S2, ESIw). Moderate
conversion (57%) in a styrene derivative (Table S2, entry 7,
ESIw) is caused by formation of by-products due to Heck
reaction. The substituted styrene can produce both Suzuki
and Heck coupling products in the presence of a base.
¨
M. A. K., R. G., and R. G. are supported by TUBITAK-
BIDEB fellowship. This work is partially supported by grants;
TUBITAK 109T603, TUBA-GEBIP, and FP7 Marie Curie
IRG. We thank M. Guler for help in TEM and Z. Erdogan for
help in LC-MS and GC-MS.
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In addition, we optimized conditions for aryl bromides in
water at room temperature. The NaOH was used as a base
(Table S1, entry 10, ESIw). The Pd@Peptide nanocatalyst
showed excellent catalytic activity with aryl bromides in water
at room temperature (Table 2). Most of the reactions were
completed with 99% yield. We also investigated the effect of
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in ethanol. The 40-methoxybiphenyl-4-ylboronic acid also
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Isolation of the catalysts from the reaction mixture and
using them in successive reactions make the chemical process
cost effective on the industrial scale and prevent accumulation
of mass palladium waste. Therefore, we further evaluated
recyclability of our catalyst under harsh conditions (in an
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.